This invention relates to a local area network (LAN) in which part at least of the network uses optical fibres as the data transmission medium.
In a local area network of the Ethernet type, which is one of the best established LAN standards, collisions, i.e. the condition which results when two or more of the system's nodes attempt to transmit at the same time, are usually detected by monitoring the signal level on the transmission medium. In the case of a metallic medium this involves monitoring the voltage level, whereas in the case of an optical fibre medium it involves monitoring the light level on the medium. To do this some indication of the attenuation condition needs to be obtained. This invention has as its object the provision of an arrangement for making such attenuation determination in respect of a system using an optical fibre medium.